Punched data tape is used for the control of communications, computing and process control apparatus. Such tape is prepared by a punch assembly which is controlled to create selected sets of holes in "data word locations" which are transverse to the length of the tape and the data content of such data word locations are determined by a reader assembly. Each such transverse set of holes comprises a parallel data word.
Paper data tape has sprocket holes throughout its length and it is advanced through the punch and reader assemblies by sprocket wheels in those assemblies. The sprocket wheels are stepped in increments such that the tape is advanced a distance equal to the distance between successive data word locations. The tape is momentarily stopped at each possible data word location to facilitate processing (punching and reading). The tape drive mechanism in the punch and reader assemblies provides timing signals for control of the punch and reader assemblies.
Paper data tape naturally has great variations in opacity and upon occasion a foreign substance, e.g. grease, contacts the tape and substantially reduces the opacity of the web. Such variation causes difficulty in accurately reading the information stored in the tape. Typically, prior art circuits utilize reference voltages which are set for a standard opacity of tape web. Such circuits have limited margins and require adjustment.